18 FAIRBANKS AND RAMPART QUADRANGLES. 
feet in thickness, but are so numerous as to form probably a consid- 
erable proportion of the entire mass. 
Although the proportion of metamorphosed igneous material is 
small, fresh granitic intrusives are rather common and occur as irreg- 
ular masses up to a few miles in diameter, fringed with dikes that 
complicate the structure of the schists. The elaborate system of 
repeated intrusion, so extensively developed in the Fortymile region, 
with its far-reaching metamorphic effects, has not extended to the 
area under consideration. 
The western limit of the metamorphics is in the ridge northwest 
of the valley of the Chatanika. This ridge is formed of quartzitic 
schists, with some carbonaceous schists and crystalline limestone. A 
few miles nearer Beaver Creek, at the ends of spurs extending later- 
ally from this ridge toward Beaver Creek, there are outcrops of less- 
metamorphosed feldspathic quartzite that is included in the later 
group. A small area of garnetiferous schist with associated crystal- 
line limestone occurs in the Rampart region on Minook Creek and is 
included in the Birch Creek schist. 
PALEOZOIC ROCKS. 
SILURIAN AND DEVONIAN. 
Between the two areas of schists above described, one the westward 
extension of the schists of the Fairbanks region, the other a small 
area whose limits are not known, lie rocks most of which are regarded 
provisionally as Silurian and Devonian. The bulk of the formation, 
as already mentioned, is made up of cherts, slates with alternating 
beds of conglomerate, finer fragmentals, limestones, and greenstones. 
These rocks may be roughly divided into two groups according as 
cherts, slates, and greenstones, or conglomeratic rocks with massive 
limestones, greenstones, slates, and quartzites predominate. It is be- 
lieved that those forming the first division are older. The term 
"Rampart series" has been used by Spurr in grouping similar 
rocks occurring in other parts of the Yukon-Tanana country. In 
those areas, however, diabase, tuffs, and green slates are most abun- 
dantly developed, and carbonaceous slates and limestones, though fre- 
quently present, are of minor importance. In the areas under con- 
sideration, while greenstones are found throughout the assemblage, 
black slates, cherts, and massive dark and light-gray limestones are 
more common and more strikingly characteristic. A section of these 
rocks is crossed in traveling from Chatanika River northward to the 
southern edge of Yukon Flats, a distance of about 60 miles, and this 
section, though incompletely studied, has afforded some information 
in regard to the structure and age of the rocks composing it. 
" Spurr, J. S., Eighteenth Ann. Rept. U. §, Geol. Survey, pt. 3, 1897, pp. 155-169., 
